BGP (BGP4 or BGP-4): Border Gateway Protocol

The Border Gateway Protocol (BGP), runs over TCP, is an inter-Autonomous System routing protocol. BGP is the only protocol that is designed to deal with a network of the Internet's size, and the only protocol that can deal well with having multiple connections to unrelated routing domains. It is built on experience gained with EGP. The primary function of a BGP system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASs) that reachability information traverses. This information is sufficient to construct a graph of AS connectivity from which routing loops may be pruned and some policy decisions at the AS level may be enforced.

BGP4 provides a new set of mechanisms for supporting classless interdomain routing (CIDR). These mechanisms include support for advertising an IP prefix and eliminates the concept of network "class" within BGP. BGP-4 also introduces mechanisms which allow aggregation of routes, including aggregation of AS paths. These changes provide support for the proposed supernetting scheme.

Protocol Structure - BGP Border Gateway Protocol<

Marker (16 byte)

Length (2 byte)

Type (1 byte)

• Marker -- Message containing a value predictable by the receiver of the message.
• Length -- The length of the message including the header.
• Type --The message type. Possible messages are: Open, Update, Notification, KeepAlive.

After a transport protocol connection is established, the first message sent by each side via BGP is an OPEN message. If the OPEN message is acceptable, a KEEPALIVE message confirming the OPEN is sent back. Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION messages may be exchanged. The format of each type of messages could be found in the reference documents

BGP (BGP4) is defined by IETF (http://www.ietf.org ) RFC 1771.

http://www.javvin.com/protocol/rfc1774.pdf : BGP-4 Protocol Analysis